Description of the Related Art
Such a drive assembly is known from DE 39 16 144 A1 where the coupling hub of the viscous coupling at the same time forms the outer part of a tripode joint. The inner part of the tripode joint, i.e. the tripode star including the rollers rotatably and sidably arranged thereon is directly associated with the output of a distributing drive. Between the distributing drive and the drive driving the wheels of the permanently driven front axle there exists a driving connection. The direct coaxial arrangement of the viscous coupling and the integration of its coupling hub into the joint are disadvantageous in that the two components influence each other. During the transmission of torque the joint heats up, with such heat being introduced into the viscous coupling directly, i.e. the viscous coupling is heated up by the joint, and vice versa, the heat developing in the viscous coupling flows into the joint.
However, to a considerable extent, the torque characteristics of the viscous coupling depend on the temperature rise of the fluid contained therein. With an increasing temperature, the viscosity of the fluid is reduced, which is followed by a reduction in the torque transmitted. As the temperature rises further, the filling level of the viscous coupling rises and thus the amount of transmittable torque. If at high absolute speeds, the viscous coupling is heated up by a joint, it is possible for the viscous coupling to transmit torque at a time when this is not desirable. With such an arrangement, the torque characteristics of the viscous coupling move into an unwanted range.
There is a further disadvantage in that the viscous coupling represents a considerable mass which, as a result of the special characteristics of the tripode joint relative to the fixed tripode star, carries out a cyclically eccentric movement around the joint centre. At high speeds, this leads to out-of-balance.